Insulated electrical conductor and method of making the same



Aug. 1, 1961 R. SAXON 2,994,623

INSULATED ELECTRICAL CONDUCTOR AND METHOD OF MAKING THE SAME;

Filed Jan. 20. 1960 COATING OF INSULATION COMPRISING A TERNARY POLYMEROF ACRYLONITRILE,

A VINYLPYRIDINE AND A THIRD DIFFERENT MONOETHYLENICALLY UNSATURATEDMATERIAL INSULATING COATING OF ORDINARY ORGANIC-TYPE ENAMEL COATING OFINSULATION COMPRISING A TERNARY POLYMER OF ACRYLONITRILE, AVINYLPYRIDINE AND A THIRD DIFFERENT MONOETHYLENICALLY UNSATURATEDMATERIAL CONDUCTOR INSULATING COATING OF ORDINARY ORGANIC-TYPE ENAMELCOATING OF INSULATION COMPRISING A TERNARY POLYMER OF ACRYLONITRILE, AVINYLPYRIDINE AND A THIRD DIFFERENT MONOETHYLENICALLY UNSATURATEDMATERIAL INSULATING COATING OF ORDINARY ORGANICTYPE ENAMEL COATING 0FINSULATION COMPRISING A TERNARY POLYMER OF ACRYLONITRILE, AVINYLPYRIDINE AND A THIRD DIFFERENT MONOETHYLENICALLY UNSATURATEDMATERIAL FIG.4

IN VEN TOR. ROBER T SAX O N A TTORNE Y United States Patent 2,994,623INSULATED ELECTRICAL CONDUCTOR AND METHOD OF MAKING THE SAME RobertSaxon, Stamford, Conn., assignor to American Cyanamid Company, New York,N.Y., 'a corporation of Maine Filed. Jan. 20, 1960, Ser. No. 3,618 14Claims. (Cl. 117-232) This invention relates to insulated electricalconductors and to a method of making the same. More particularly, theinvention is concerned with the manufacture of insulated electricalconductors having insulation of the organic-enamel type thereon.

In accordance with the present invention, metallic conductors, e.g.,copper wire, aluminum wire, etc., are insulated with a particularternary polymer. More specifically, the insulation on the conductorcomprises a ternary polymer of, by weight, from 80% to 96%acrylonitrile, from 2% to of a vinylpyridine and from 2% to 10% of athird, difierent, monoethylenically unsaturated material. The insulationcoating is hard, flexible, moistureand abrasion-resistant and has a highdielectric strength. It is also resistant to attack by such ingredientsas oils, solvents, varnishes, and various chemicals.

It was known prior to the present invention to use a variety of resinousand polymeric materials dissolved in a suitable solvent as a so-calledwire enamel or coating material in place of, or in addition to, thevarious enamels of the conventional varnish type that have been used formany years in the electrical industry. These older types of insulatingcoatings have been generally described as oleo-resinous coatingcompositions or enamels. Such oleo-resinous varnishes are not entirelyresistant to mineral oil and mineral-oil-containing materials, and hencehave been replaced to a large extent in recent years by wire enamelscontaining various synthetic resinous and polymeric materials.

One of the most eflective and widely used wire enamels of the syntheticresin type is that described in Patent No. 2,085,995 and a modified formthereof in Patent No. 2,307,588. The first-named patent describesinsulated conductors wherein the insulation is a polyvinyl acetal resin,specifically a polyvinyl formal resin, which is produced by condensingan aldehyde, e.g., formaldehyde, with a product of hydrolysis of apolyvinyl ester, e.g., polyvinyl acetate. The last-named patent involvesa wire enamel comprising a polyvinyl acetal resin of the kind justdescribed, which resin has been modified with a phenol-aldehyde resin,specifically a cresol-formaldehyde resin.

Other patents involving the use of wire enamels containing a syntheticresinous or polymeric material are Patent No. 2,271,233, directed to theuse of a phenolaldehyde resin-modified superpolyamide, by which is meantspecifically a synthetic linear condensation prodnet of the kinddescribed in British Pats. 461,236 and 461,237, and now knowngenerically as nylon; and Patents 2,191,584 and 2,249,498. In both ofthese last-named patents reference is made to the use of certainsynthetic materials of the polymerized acrylic compound. or polymerizedacrylic acid types. By this language the respective patentees includepolymerized methyl and ethyl esters of acrylic acid, polymerizedacrylonitrile, separately or conjointly polymerized mixtures of methyland ethyl esters of acrylic acid and separately or conjointlypolymerized mixtures of either or both of the aforesaid esters withacrylonitrile.

The novel features of my invention are set forth in the appended claims.The invention itself, however, will be understood most readily from thefollowing more de tailed description when considered in connection withthe accompanying drawing in which:

FIG. 1 is a cross-sectional view of an electrical conductor providedwith insulation in accordance with the present invention;

FIG. 2 is also a cross-sectional view showing the conductor providedwith a coating of insulation of ordinary organic-type enamel and asuperimposed coating of the insulating material hereinafter more fullydescribed;

FIG. 3 is another cross-sectional view illustrating a furthermodification of the invention; and

FIG. 4 is also a cross-sectional view showing a still furthermodification of the invention.

The present invention is based on my discovery that a ternary polymer ofthe kind broadly described in the second paragraph of this specificationand more fully hereinafter provides an improved electrically-insulatingcoating on electrical conductors that permits the construction of morecompact electrical equipment or, alternatively, more powerful equipment,e.g., motors, of about the same size than heretofore has been possiblewith the oleo-resinous wire enamels of many of the available wireenamels containing a synthetic resin or polymer. This is because theinsulating materials used in practicing the present invention have ahigh degree of temperature resistance, particularly for a thermoplasticpolymer; and, therefore, heat buildup due to passage of electric currentthrough the conductor during the use of the apparatus containing theinsulated conductors of this invention does not cause failure of theinsulation.

It was quite surprising and unexpected that a thermoplastic polymershould give such good heat resistance, while maintaining all of theother desirable properties of a wire enamel since, by their inherentnature, thermoplastic polymers are subject to flow or deformation. Theimproved and unexpected results obtained from the insulated conductorsof the present invention are due in part to the presence of a relativelysmall but critical amount of a vinylpyridine, more particularly an alkylvinylpyridine, and specifically 2-methyl-5-vinylpyridine, as anessential ingredient of the ternary polymer component of the wireenamel; and to the use of a solvent comprising essentiallydimethylformamide as the volatile (volatilizable) solvent employed inthe wire enamel.

In order that those skilled in the art may better understand how thepresent invention can be carried into effect, the following examples aregiven by way of illustration and not by Way of limitation.

EXAMPLE 1 Preparation of ternary polymer A ternary polymer is made froma monomeric mixture of 7.5% vinyl acetate, 7.5 2-methyl-5-vinylpyridineand acrylonitrile by continuously polymerizing the aforesaid mixture inan aqueous medium using an oxidationreduction catalyst system comprisedof chloric acid and sulfuric acid and following the general proceduredescribed in Mallison U.S. Pataent No. 2,777,832. The resulting ternarypolymer contains, in combined state, about 86.0% acrylonitrile, about6.3% 2-methyl-5-vinylpyridine and about 5.4% vinyl acetate. This ternarypolymer is dried and made into a wire enamel as described in Example 2.

EXAMPLE 2 Preparation of wire enamel One hundred and sixty parts of thedried, finelydivided ternary polymer of Example 1 is dissolved in 840parts of dimethylformamide to form a wire enamel. This wire enamel isused in producing an insulated electrical conductor as described inExample 3.

EXAMPLE 3 Preparation of insulated electrical conductor The wire enamelof Example 2 is used to coat No. 17 A.W.G. copper wire at a speed of 17feet per minute in a wire-coating tower held at about 300 C. Theso-called build, that is, the increase in wire thickness as a result ofthecoating operation, is 29 mils. A smooth coating is formed on thewire. The coated wire successfully passes a flexibility test in which,after the coated wire has been given a 35% stretch, it is wound around amandrel which is the same diameter as the wire. The so-called scrapehardness is 63, this being the number of ounces of load on a 9-mildiameter knife-edge to scrape enamel from the wire. The coated wire alsoshows excellent abrasion resistance, as evidenced by the fact that, inorder to cause failure, from 78 to 186 strokes with a 16-mil knife-edgeunder a 740 g. load are required. In a heat-shock test, the coated wireis wound around its own diameter, heated at 200 C. for 15 minutes, andthen cooled. Under this test, the coated wire does not show cracking orother evidence of failure of the insulation. The insulated electricalconductor shows a dielectric breakdown, in duplicate tests, of 3.0 and4.0 k.v.

Instead of the ternary polymer made as described in Example 1, one canuse a terpolymer made from a monomeric mixture of 7.5% methyl acrylate,7.5 Z-methyl- -vinylpyridine and 85% acrylontrile which has beenproduced as broadly descriped in Example 1 and in the pataent to whichreference is there made. The resulting ternary polymer contains, incombined state, about 85% acrylonitrile, about 5.4%2-methyl-5-vinylpyridine and about 7.5% methyl acrylate. This ternarypolymer is made into a wire enamel as described in Example 2, whichenamel is then used in making an insulated, coated wire as described inExample 3. The properties of the resulting insulated electricalconductor are much the same as the properties of the insulatedelectrical conductor of Example 3.

Various other ternary polymers comprising acrylonitrile, avinylpyridine, and a third, different, monoethylenically unsaturatedmaterial, using the ranges of proportions of these ingredients that aregiven in the second paragraph of this specification, can be used insteadof the two specific ternary polymers described hereinbefore. A preferredsub-class of ternary polymers that are useful in practicing the presentinvention is the one which comprises ternary polymers of, by weight,from 80% to 90% acrylonitrile, from 5% to of a vinylpyridine including2-methyl-5-vinylpyridine, and from 5% to 10% of a vinyl ester,specifically vinyl acetate. Additional examples of ternary polymersfalling within this preferred sub-class are those made from a monomericmixture of, by weight,

(a) 84% acrylonitrile, 8% acrylamide and 8% Z-methyl- 5-vinylpyridine,'

(b) 90% acrylonitrile, 5% methyl methacrylate and 5%2-methyl-5-vinylpyridine,

(c) 86% acrylonitrile, 7% methacrylonitrile and 7% 2-vinyl-5-ethylpyridine (or 7% 2-methyl-5-vinylpyridine).

by the formula L CH=CHi and which include 2- vinylpyridine,3-vinylpyridine, and

( CH=CH2 and which include 2-methyl-3-vinylpyridine, 3-vinyl-4-methylpyridine, 3 vinyl 5 methylpyridine, 2-vinyl-3- methylpyridine, 2vinyl 4 methylpyridine, 2-vinyl-5- methylpyridine, 2 vinyl 6methylpyridine, 2-methyl-4- vinylpyridine and 3-methyl-4-vinylpyridine.The vinylpyridines embraced by Formula II are a preferred subgroupwithin a broader class of vinylpyridines that are desirably employed inmaking ternary polymers for use in practicing the present invention andwhich may be represented by the formula (III) CH=CH2 and wherein Rrepresents a lower alkyl radical, more particularly a methyl, ethyl,propyl (including n-propyl and isopropyl) or butyl (including n-butyl,isobutyl, sec.- butyl and tert.-butyl) radical. Other examples include2-vinyl-4,6-dimethylpyridine, and 2- and 4-vinylquinolines,2-vinyl-4,6-diethylpyridine and others embraced by the formula I CH=CH1wherein R represents a lower alkyl radical, examples of which have beengiven hereinbefore, and n represents an integer from 1 to 5, inclusive.One can substitute in the specific ternary polymers mentionedhereinbefore an equivalent amount of any of the vinylpyridines, of whichnumerous examples have just been given, for the Z-methyl-S-vinylpyridine employed in the previously given formulations.

Illustrative examples of other comonomers with acrylonitrile and avinylpyridine that can be used as the third, different,monoethylenically unsaturated material are, for example, vinyl estersincluding the formate, propionate, butyrate; the various acrylic estersincluding the lower alkyl acrylates and methacrylates such as the ethyl,propyl, and butyl acrylates and the methyl, ethyl, propyl, and butylmethacrylates; the various acrylamides including acrylamide itself andmethacrylamide; the various acrylic acids including acrylic acid itselfand methacrylic acid; methacrylonitrile and other copolymerizablesubstituted acrylonitriles; unsaturated alcohols including allylalcohol; vinyl-substituted aromatic hydrocarbons, e.g., styrene, thevarious ring-substituted methylstyrenes; isopropenyl toluene; and othersincluding those given by way of example in, for instance, Cresswell US.Patent No. 2,558,730, dated July 3, 1951 (column 3, lines 31-55), andPrice US. Patent No. 2,736,722, dated February 28, 1956 (column 4, line66 through line 27 in column 5). The third difierent monoethylenicallyunsaturated material mentioned above includes within its meaning aplurality of such materials. Advantageously the third, different,monoethylenically unsaturated material is a compound which contains a CH=C grouping.

The-ternary polymers employed in making the wire enamels, which then areused in making the insulated electrical conductors of the presentinvention, ordinarily have a molecular Weight (average molecular weight)within a fiber-forming (fiber-formable) range; that is to say, withinthe range of from about 15,000 or 20,000 to about 200,00 as calculatedfrom a viscosity measurement of the ternary polymer in dimethylformamideusing the Staudinger equation (reference: Houtz US. Patent No.2,404,713, dated July 23, 1946). The use of ternary polymers having anaverage molecular weight above 200,000, e.g., about 300,000 and higher,is, of course, not precluded. Ordinarily the average molecular weight ofthe ternary polymer employed is within the range of from about 50,000 toabout 100,000, and more specifically within the range of from about60,000 to about 90,000.

Optimum results have been obtained by using a solvent comprising orconsisting substantially completely of dimethylformamide for dissolvingthe ternary polymer in order to form the wire enamel. The concentrationof the ternary polymer in the solvent comprised (or composed solely) ofdimethylformamide may be considerably varied but generally is the rangeof from about 5 to about 25 parts ternary polymer to from about 95 toabout 75 parts of the aforesaid solvent. A preferred range within thisbroader range is one wherein the ratio of ternary polymer is from aboutto about 20 parts of ternary polymer to from about 90 to about 80 partsof solvent comprised or consisting essentially of dimethylformamide.

The active or primary solvent, specifically dimethylformamide, may beused in combination with minor amounts (less than 50% by weight) ofother solvents for the terpolymer including those described in, forexample, U.S. Patents 2,404,714 to 2,404,728, inclusive. This may havecost advantages (without adversely afiecting the properties of theinsulated conductor) when the added solvent is less expensive thandimethylformamide.

Alternatively, the dimethylformamide solvent or mixture thereof with aminor amount, e.g., 5-40% by weight, of a difierent solvent for theterpolymer may be diluted with a diluent (e.g., acetone) which isnormally a nonsolvent for the terpolymer, the amount employed beinginsuflicient to cause precipitation of the terpolymer. Such amounts mayvary widely depending upon the particular solvent and non-solventcombination, but ordinarily the non-solvent is less than 50% by volumeof the total volume of solvent plus non-solvent. Dilution of this naturehas the advantage of lowering the cost of applying the wire enamel.Examples of diluents, in addition to acetone, that can be used arephenol and cresol.

Enameled wires are produced by drawing the clean wire, for example cleancopper wire, through a bath of wire enamel made as described above.Thereafter the coated wire is subjected to heat, for instance byintroducing it into a suitable oven or chamber wherein the appliedcoating is baked at a suitable temperature. With dimethylformamide aloneas the solvent, a suitable temperature is about 250300 C. With mixturesof dimethylformamide and a different solvent for the terpolymer and/ ora diluent of the kind and in the amount described above, the temperaturefrequently will be outside of this range. In all cases the temperatureshould be high enough to volatilize the liquid component(s) of the wireenamel but not so high as to cause blistering of the coating as a resultof rapid evaporation.

The coating is baked simultaneously with the annealing of the copper.Usually it is necessary to run the wire successively through the enamelbath and baking oven several times in order to provide adequateinsulation thereon. Baking improves the insulating properties of thecoating, and may be carried out at any elevated temperature up to thepoint of incipient decomposition of the ternary polymer. In all cases,the higher the temperature employed, the shorter the period of heating;and, conversely, the lower the temperature, the longer the heating time.The baking treatment results in conversion of the coating on the wire toa hard, tough, abrasionand moisture-resistant state.

It is to be understood that this invention is not limited to theapplication of the wire enamel, used in making the insulated conductorsof this invention, directly upon the conductor as shown in FIG. 1. Forexample, a coating of the insulation may be applied over a coating ofregular enamel as shown in FIG. 2. The ternary polymer compositionadheres tenaciously to the underlying enamel film, and protects thelatter from abrasion and from embrittlement which otherwise results uponprolonged exposure to heat. Also, if desired, a conductor may beprovided first with a coating of the ternary polymer, followed by one ormore coatings of ordinary enamel as shown in FIG. 3. In this Way theadherence of the ordinary organic enamel is improved. Thereafter anouter coating or coatings of the ternary polymer wire enamel may beapplied to produce an insulated electrical conductor as shown in FIG. 4.

Wires insulated with the Wire enamels of this invention are far superiorto those insulated with conventional organic enamels. The heat-treatedcoating as present on the wire is not only extremely hard and tough, butit is also flexible. The coating on a copper conductor is, in general,more flexible than the copper itself, as evidenced by the fact that itis possible to stretch a coated copper wire of thist invention to thebreaking point and still wind the Wire on itself without evidence ofcracking of the film or coating. The film is resistant to almost allcommonly used solvents, e.g., petroleum hydrocarbon naphthas, alcohols,and aromatic solvents such as benzene, toluene, xylene, and the like.

The insulation coatings on electrical conductors produced ashereinbefore described are also exceptionally resistant to heat aging.For example, tests indicate that at C. the coated wires of thisinvention will not lose their extreme flexibility when heated for aslong as 2,000 hours at 105 0, whereas ordinary organic-type wire enamelsgenerally will not withstand heating for longer than about 24 hours at105 C. without cracking on a mandrel test such as has previously beendescribed, Insulated Wires of this invention can be elongated to anyextent necessary in practical manufacturing, treated with varnish, andimmediately placed in an oven at a temperature of C.

The dielectric strength of the insulation of the insulated conductors ofthis invention is high, and the dielectric losses are low. It is,therefore, possible to use a much thinner overall insulation thanpreviously had been required with most of the prior Wire enamels.Because less insulation is required, more copper can be placed in theslot of a motor or generator, resulting in an increased rating of themachine. This applies to transformers, regulators, and other electricalapparatus in which insulated wire is used in the form of a coil, On theother hand, if the rating of the machine is kept constant, there resultsa material saving of copper and iron.

The insulated electrical conductors of this invention are useful inelectrical motors, magnets, coils, and other electrical devices whereinan insulated conductor, such as copper, aluminum or other conductingmetallic wire, is tightly wound or packed. The insulated conductors maybe used in cables and transmission lines, or for general electricalpurposes.

I claim:

1. An insulated electrical conductor in which the insulation comprises aternary polymer of, by weight, from 80% to 96% acrylonitrile, from 2% to10% of a vinylpyridine and from 2% to 10% of a third, different, monoethylenically unsaturated material.

2. An insulated electrical conductor as in claim 1 wherein the ternarypolymer has an average molecular weight within the range of from about50,000 to about 100,000.

3. An insulated electrical conductor as in claim 1 whrerein thevinylpyridine component of the ternary polymer of which the insulationis comprised is a vinylpyridine including Z-methyl-S-vinylpyridine.

4. An insulated electrical conductor as in claim 1 wherein a vinyl esteris the third, different, monoethyl- '7 enically unsaturated materialwhich is present in the ternary polymer of which the insulation iscomprised.

5. An insulated electrical conductor in which the insulation comprises aternary polymer of, by weight, from 80% to 90% acrylonitrile, from to ofa vinylpyridine including 2-methyl-S-vinylpyridine, and from 5% to 10%of vinyl acetate, said ternary polymer having an average molecularweight within the range of from about 60,000 to about 90,000.

6. An insulated electrical conductor as in claim 5 wherein theelectrical conductor is copper wire.

7. An electrical conductor having superimposed directly thereon a hard,flexible, tough, abrasion-resistant coating of high dielectric strength,said coating being a heat-treated ternary polymer having the compositiondefined in claim 1. 8. An electrical cable comprising an electricalconductor provided with a hard, flexible, tough, abrasionresistantcoating of high dielectric strength, said coating being the heat-treatedternary polymer having the composition defined in claim 5.

9. The method which comprises dissolving in a solvent comprisingdimethylformamide a ternary polymer of, by weight, from 80% to 96%acrylonitrile, from 2% to 10% of a vinylpyridine and from 2% to 10% of athird, different, monoethylenical ly unsaturated material, said ternarypolymer having an average molecular weight within the range of fromabout 50,000 to about 100,000 and being dissolved in said solventcomprising dimethylformamide in an amount that willprovide a solutionhaving a ratio of ternary polymer to solvent of, by weight, from about 5to about 25 parts ternary polymer to about 95 to about 75 parts solvent;applying the resulting solution to an electrical conductor to form acoating thereon; and heating the thusly coated electrical conductor at atemperature sufliciently high to volatilize the solvent and to form ahard, flexible, tough, abrasionresistant coating of high dielectricstrength on said conductor. t L

' 10. A method as in claim 9 wherein the ternary polymer is one whichhas the composition defined in claim 5, the ratio of ternary polymer tothe solvent comprising dimethylformamide is, by weight, from about 10 toabout 20 parts of ternary polymer to about 90 to about parts of saidsolvent, and the electrical conductor is copper wire. t

11. A method as in claim 9 wherein the solvent consists substantiallycompletely of dimethylformamide.

12. Copper wire in the form of a coil and which is insulated withelectrical insulation comprising a ternary polymer of, by weight, from80% to 96% acrylonitrile, from 2% to 10% of a vinylpyridineand from 2%to 10% of a third, different, monoethylenically unsaturated material,said ternary polymer having an average molecu* lar weight within therange of from about 50,000- to about 13. Copper magnet wire havingsuperimposed directly thereon a coating of a ternary polymer of, byweight, from 80% to acrylonitrile, from 5% to 10% of a vinylpyridineincluding 2-methyl-5-vinylpyridine, and from 5% to 10% vinyl.acetate,,said ternary polymer having an average molecular weight withinthe range of from about 60,000 to about 90,000. t v v 14. Electricalapparatus whereinfan insulated metallic conductor is tightly wound, theinsulation on said conductor comprising a ternary polymer of, by weight,from 80% to 96% acrylonitrile, from 2% to 10% of a vinylpyridine andfrom 2% to 10% of a third, different, monoethylenically unsaturatedmaterial. a,

References Cited in the file of this pa tent UNITED STATES PATENTS2,658,879 Beaman Nov. 10, 1953 2,891,035 Price June 16, 1959 UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,994,623August 1, 1961 0 Robert Saxon a 7 It is hereby certified that errorappears in the above numbered patent requiring correction and that thesaid Letters Patent should read as corrected below.

Column 3, line 28, for "acrylontrile" read aorylonitrile Y line 29, for"descriped" read described line 30, for "pataent" read patent column 1,line 75, for "200,00" read 200,000 column 6, line 22, for thist" readthis Signed and sealed this 5th day of December 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer I 0 Commissioner ofPatents USCOMM-DC

9. THE METHOD WHICH COMPRISES DISSOLVING IN A SOLVENT COMPRISINGDIMETHYLFORMAMIDE A TERNARY POLYMER OF, BY WEIGHT, FROM 80% TO 96%ACRYLONITRILE, FROM 2% TO 10% OF A VINYLPYRIDINE AND FROM 2% TO 10% OF ATHIRD, DIFFERENT, MONOETHYLENICALLY UNSATURATED MATERIAL, SAID TERNARYPOLYMER HAVING AN AVERAGE MOLECULAR WEIGHT WITHIN THE RANGE OF FROMABOUT 50,000 TO ABOUT 100,000 AND BEING DISSOLVED IN SAID SOLVENTCOMPRISING DIMETHYLFORMAMIDE IN AN AMOUNT THAT WILL PROVIDE A SOLUTIONHAVING A RATIO OF TERNARY POLYMER TO SOLVENT OF, BY WEIGHT, FROM ABOUT 5TO ABOUT 25 PARTS TERNARY POLYMER TO ABOUT 95 TO ABOUT 75 PARTS SOLVENT,APPLYING THE RESULTING SOLUTION TO AN ELECTRICAL CONDUCTOR TO FORM ACOATING THEREON, AND HEATING THE THUSLY COATED ELECTRICAL CONDUCTOR, ATA TEMPERATURE SUFFICIENTLY HIGH TO VOLATILIZE THE SOLVENT AND TO FORM AHARD, FLEXIBLE, TOUGH, ABRASIONRESISTANT COATING OF HIGH DIELECTRICSTRENGTH ON SAID CONDUCTOR.